The expectation for the development of energy saving technology by controlling the frictional force has increased as the global environmental and energy problems get worse. For example, as a familiar example, there are a large number of drive units in the interior of an apparatus such as a generator, and energy loss due to friction occurs at the respective places thereof. In order to suppress this problem, it is effective to decrease the friction between the materials.
Hitherto, there has been a technology to decrease the frictional force by coating the existing structural materials. The frictional property of this coating is greatly changed by changes in the crystal orientation, structure, and composition thereof, and thus how efficiently investigates the crystal orientation, structure, and composition which exhibit low frictional property in a short period of time has been the key to research and development.
As a material for performing such coating, it is desired to use resources which are abundant and inexpensive and have small environmental impacts. For example, the inventors of the present invention and the like have found out and disclosed that zinc oxide which is an oxide of zinc (Zn), one of metals satisfying this condition exhibits a low friction phenomenon when the crystal orientation thereof is optimized. In addition, zinc oxide is a piezoelectric material, and the inventors of the present invention and the like also have found that this piezoelectric property contributes to a decrease in friction (Patent Literature 1, Non Patent Literatures 1 to 4).
However, the specific mechanism of achieving low friction by piezoelectric property to be considered to affect the frictional property of ZnO coating has not yet been satisfactorily investigated.
In addition, the research and development of micromachines (also referred to as micro electro mechanical system (MEMS)) have been progressed in recent years. It has been found that the friction between mechanical members (for example, the friction between minute gears) in a micromachine greatly affects the mechanical performance of the micromachine. The size of the contact region between the mechanical members in a micromachine is in a nanometer order in some cases, and the friction occurring in such a region in a nanometer order is also often different from the macroscopic friction measured in the region in a millimeter order in facet. However, sufficient researches have not been carried out on the decrease in friction in the region in a nanometer order of zinc oxide coating.